1. Field of the Invention
This invention relates to receivers for mobile telephone systems.
With the impending adoption of the GSM (Groupe Speciale Mobile) system for a Pan-European mobile Telephone System (see for example Electronics and Communication Engineering Journal Jan./Feb. 1989 vol. 1 no. 2 pp. 7 to 13, "Pan-European cellular radio", D. M. Balston), a number of difficult technical problems have to be overcome for satisfactory implementation of the system.
In the GSM system, data to be transmitted is modulated on to a 900 MHz carrier by a Gaussian minimum shift keying technique (GMSK). Frequency channels are provided at a spacing of 200 KHz, and data is transmitted on each channel at a total rate of 270.833 kbits.sup.-1.
2. Description of the Prior Art
Referring to FIG. 1, this shows the basic format of one frequency channel for GSM transmissions as comprising a series of frames (Time Domain Multiple Access Frames), each frame comprising eight multiplexed time slots from different mobile stations. Each frame is 4.615 ms in length and each time slot 0.577 ms in length. The structure of a time slot as shown comprises two sections of data separated by control bits, tail bits etc. Data is transmitted from each mobile station within a time slot in compressed format at a rate of approximately 13 kbits.sup.-1.
Given the very tight technical constraints of the GSM system, interference and data loss and corruption can easily occur.
Problems arise in the receiver for the GSM terminal, whether it be a terminal in a mobile station or in a base station. Firstly, the strength of the signal received by the terminal will vary and the input dynamic range may be as large as 100 dB. This is because, in the case of a mobile station, it must not only receive data signals from the adjacent base station but must also monitor neighboring base stations during time slots of the frame not reserved for data transmission for hand over purposes. Hence it is necessary to adjust the gain by very large amounts from time slot to time slot, i.e. within 0.577 ms. Conventional methods of automatic gain control can not cope with such extreme demands. Secondly, received signals at whatever level they are received must be amplified to a suitable level so that they can be accommodated within the input dynamic range of an analog to digital converter (ADC) system which follows the IF stage. Since the dynamic range of an ADC is relatively small, it is necessary to adjust the gain of the system so that each incoming pulse of RF energy is at an appropriate level.
It is proposed in accordance with the invention disclosed in co-pending U.K. Patent Application No. GB2233846A to provide a receiver for a mobile telephone system operating in a time division multiple access (TDMA) system, the receiver having a gain control means coupled to receive an input signal and being switchable under the control of digital control signals between a plurality of gain and/or attenuation levels, equalizer means coupled to receive the output signal from the gain control means and means for determining the value of the input signal and for providing said digital control signals to fix the gain or attentuation level constant for a time slot of the TDMA system, the period, the determining means including means for recording previous values of the input signal to fix the gain level prior to said time slot based upon an assessment of said previous values.
The receiver disclosed and claimed in co-pending U.K. Patent Application No. GB2233846A operates satisfactorily for traffic channels, and is intended for use in a GSM mobile terminal. However, whilst similar problems occur in a base station receiver, further problems arise in addition requiring a different approach to AGC control.
Problems arise in particular in the base station when a mobile station wishes to gain access in order to initiate a call. In order to gain access, the GSM system provides a random access channel (uplink from the mobile to the base station) which is used by the mobile to request access. An access grant channel (downlink only) is also provided, which replies to a random access and assigns a dedicated control signal for subsequent signalling. The structure of a random access channel is shown in FIG. 2, as comprising a start sequence of eight bits, followed by a fixed preamble for the equalizer section of the base station, a data I.D. section, and a long guard period. Such a guard period is necessary since the mobile may not be synchronized with the base station, and since the mobile may be as far as 35 km from the base station, there may be substantial propagation delays (240 microsecond loop delay) so that the data may arrive at the base station at any time within the random access channel slot. When an access request is detected by the base station, the base station can reply on the access grant channel to correct the timing of the mobile and assign a dedicated control signal.
Problems arise however since the power level of the access request at the base station receiver antenna is unknown and may have a wide variation between -104 dB milliwatt and 0 dB milliwatt. The power level must be brought very quickly within the range of an ADC, to a central region of a 30 dB window for the ADC. In addition, more than one mobile may request access during a single random access channel, in which case it is necessary to resolve the contention between the requests.